Implement a cache for metrics

[LouisJenkinsCS]

A lot of the times, metrics are reused and can therefore be recycled when used later. It would be nice to have something like, say, memcached which maps keys to values in a way that is relatively efficient. That way it be something like...

const componentsKey = "connected components";
var components : ComponentsMetric;
if metrics.contains(componentsKey) {
   components = metrics.get(componentsKey);
} else {
   components = getComponents();
   metrics.put(componentsKey, components);
}

That can be relatively useful in the future and even in the present, as testing on DNS data commonly requires metrics to be passed around, which is very awkward right now.

[github-actions]

This issue is stale and should either be closed or eventually resolved.